1. Field of the Invention
The present invention relates to an electrode assembly and a lithium ion secondary battery using the same, and more particularly, to an electrode assembly and a lithium ion secondary battery capable of preventing a short circuit from being created in an outer peripheral portion of the electrode assembly.
2. Description of the Prior Art
As is generally known in the art, secondary batteries are different from primary batteries in that secondary batteries can charge and discharge electric power. Secondary batteries have been extensively used in advanced electronic technology fields for portable electronic appliances, such as portable phones, notebook computers and camcorders.
Particularly, lithium ion secondary batteries represent an operational voltage of about 3.7V, which is three times higher than that of Ni—Cd batteries or Ni-MH batteries used as power sources for portable electronic appliances. In addition, the lithium ion secondary batteries have high energy density per unit weight, so the lithium secondary batteries are extensively used in the advanced electronic technology fields.
In general, lithium ion secondary batteries include lithium-based oxides as positive electrode active materials and carbon materials as negative electrode active materials. In addition, secondary batteries are classified into liquid electrolyte batteries and high polymer electrolyte batteries according to the electrolytes used for the secondary batteries. The secondary batteries using the liquid electrolyte are called “lithium ion secondary batteries” and the secondary batteries using the high polymer electrolyte are called “lithium polymer secondary batteries”. In addition, the lithium ion secondary batteries can be formed with various shapes, such as cylinder type lithium ion secondary batteries, can type lithium ion secondary batteries and pouch type lithium ion secondary batteries.
As shown in FIGS. 1 and 2, the typical can type lithium ion secondary battery includes a can 10, an electrode assembly 20 accommodated in the can 10, and a cap assembly 70 for sealing an upper opening section of the can 10.
The can 10 is made from a metal having a hexahedral shape and acts as a terminal. The can 10 includes an upper opening section 10a through which the electrode assembly 20 is accommodated in the can 10.
Referring to FIG. 2, the electrode assembly 20 includes a positive electrode plate 30, a negative electrode plate 40, and a separator 50. The positive electrode plate 30 and the negative electrode plate 40 are wound in the form of a jelly-roll while interposing the separator 50 therebetween.
The positive electrode plate 30 includes a positive electrode collector 32 made from a laminated aluminum foil and a positive electrode active material layer 34 including lithium-based oxides coated on inner and outer surfaces of the positive electrode collector 32. The positive electrode collector 32 is formed with a positive electrode uncoated area 32a, in which the positive electrode active material layer 34 is not coated, corresponding to both ends of the positive electrode plate 30. A positive electrode tap 36 is fixed to the positive electrode uncoated area 32a by means of ultrasonic welding in such a manner that an end of the positive electrode tap 36 can upwardly protrude beyond an upper end of the positive electrode collector 32. The positive electrode tap 36 is generally made from Ni or a Ni-alloy. However, it is also possible to fabricate the positive electrode tap 36 by using other metallic materials.
The negative electrode plate 40 includes a negative electrode collector 42 made from a laminated aluminum foil and a negative electrode active material layer 44 including carbon materials coated on inner and outer surfaces of the negative electrode collector 42. The negative electrode collector 42 is formed with a negative electrode uncoated area 42a, in which the negative electrode active material layer 44 is not coated, corresponding to both ends of the negative electrode plate 40. A negative electrode tap 46 is fixed to the negative electrode uncoated area 42a by means of ultrasonic welding in such a manner that an end of the negative electrode tap 46 can upwardly protrude beyond an upper end of the negative electrode collector 42. The negative electrode tap 46 is generally made from Ni or a Ni-alloy. However, it is also possible to fabricate the negative electrode tap 46 by using other metallic materials.
The separator 50 is interposed between the positive electrode plate 30 and the negative electrode plate 40 so as to insulate the positive electrode plate 30 from the negative electrode plate 40. The separator 50 is made from polyethylene, polypropylene, or composition of polyethylene and polypropylene. In one exemplary embodiment, the separator 50 has a width larger than that of the positive electrode plate 30 and the negative electrode plate 40 in order to effectively prevent a short circuit between the positive electrode plate 30 and the negative electrode plate 40.
The cap assembly 70 includes a cap plate 71, an insulative plate 72, a terminal plate 73 and a negative electrode terminal 74. The cap assembly 70 is accommodated in a separate insulative case 79 and is coupled with the upper opening section 10a of the can 10 so as to seal the can 10.
However, referring to FIG. 2, the positive electrode tap 36 of the electrode assembly 20 is overlapped with the positive and negative electrode active material layers 34 and 44 of the positive and negative electrode plates 30 and 40 in a widthwise direction of the electrode assembly 20, so the thickness of the electrode assembly 20 becomes uneven widthwise along the electrode assembly 20. That is, as can be understood from a graph shown in FIG. 3, thickness variation may significantly occur in the widthwise direction of the electrode assembly 20. In particular, a left part of the graph shows a great increase of the thickness of the electrode assembly 20 relative to other parts thereof. This is because the positive electrode tap 36 may be in the left part together with the positive and negative electrode active material layers 34 and 44 of the positive and negative electrode plates 30 and 40. In this case, it is difficult to uniformly wind the electrode assembly 20 in a compact size so that the electrode assembly 20 accommodated in the can 10 cannot possess optimum volume. Accordingly, it is difficult to increase energy density of the secondary battery.
In addition, as energy density of the lithium ion secondary battery increases, heat is increasingly generated from the can during the overcharge/over-discharge or the short circuit between electrodes. Particularly, welding sections of the negative electrode plate 40 and the positive electrode plate 30 for the negative electrode tap 46 and the positive electrode tap 36 may be bonded with hetero-metal, internal resistance is increased in the welding sections of the negative electrode plate 40 and the positive electrode plate 30 so that the welding section generates a large amount of heat. If heat is generated in the vicinity of the electrode tap, the separator for insulating the positive electrode plate from the negative electrode plate may melt and shrink. In particular, a part making contact with the positive electrode tap shown in FIG. 2 generates a great amount of heat so an end portion of the separator positioned adjacent to the positive electrode plate may be significantly shrunk. In an extreme case, the separator aligned between the negative electrode plate and the positive electrode plate disappears. In this case, a short circuit could result between the positive electrode plate and the negative electrode plate.